Along both the development of radar and its ability to detect targets came the problem of identification of the target. Several identification systems have been widely used. Most noteworthy of these systems are the Mark X (SIF) and, more recently, the Mark XII. The advent of jet aircraft with a relatively small reflective area reduced the range of detection by radars to a degree unacceptable for operational use. To provide for the tracking of friendly aircraft, Mark X IFF, a beacon system, was used on a continuous interrogation basis to track those aircraft so equipped. The capability to distinguish xe2x80x9cfriend from friendxe2x80x9d was provided by an addition to the basic Mark X system, the Selective Identification Feature (SIF). The SIF addition provided reply-path coding on operating Modes 1, 2 and 3.
Mode 4 is the only crypto-secure identification mode available today. However, the reply does not provide unique airframe identification, so that it is impossible to identify friend-from-friend when only this mode has to be employed.
This invention is a modification to the present Mark XII-type, military radar/IFF equipment to enable the transponder to reply to a Mode 4 interrogation signal with its Mode 2 unique airframe identification signal in addition to the Mode 4 response. The transponder is modified by adding an AND gate and a delay means, the Mode 4 response from the computer being fed to the AND gate and passing through only when the I/P switch is activated. The output of the AND gate is delayed till the Mode 4 response is transmitted, and then fed to the encoder which thereupon is activated to supply a Mode 2 unique identification signal to be transmitted.
The ground equipment is modified by adding the equipment needed to enable the computer presently used to faithfully reproduce the entire received signal. The proper time decoding for the new, combined Mode 4 signal is not now provided.
An object of this invention is to provide presently used military IFF equipment, such as the AN/TPQ-27 IFF system and the airframe transponder used therewith, with the capability of sending and receiving a unique identification signal when operating in the crypto-secure mode.